1. Field of the Invention
The invention relates to a method for the manufacture of formable steel strip with a thickness of between 0.5 and 1.5 mm. Wide strip may be called steel sheet, but in this specification, the term "strip" only is used for convenience. One example of this strip is a product which is suitable for making the external parts of automobile structure. The invention also relates to apparatus for carrying out this method.
2. Description of the Prior Art
In the production of thin steel strip, conventionally the starting material is thick steel slab, having a thickness of between 150 and 300 mm, which after being heated and homogenized at a temperature between 1000.degree. C. and 1250.degree. C. is roughened down to form an intermediate slab with a thickness of approximately 35 mm, which is then reduced to a thickness of between 2.5 and 4 mm in a hot strip finishing train consisting of several mill stands. Further reduction to strip with a thickness of between 0.75 and 2 mm then takes place in a cold rolling installation. The previously pickled strip is cold reduced in a number of interlinked mill stands, with addition of a cooling lubricant. Methods have also been suggested in which thin slabs are cast, and after being heated and homogenized, are passed direct to a hot strip finishing train.
All such known and proposed rolling processes have been developed for discontinuous rolling operations. The casting of the slabs, the hot rolling of the slabs and the cold rolling of strip take place in different installations, which are effectively used only during a part of the available machine time. In a discontinuous rolling operation, it is necessary for the running of the installations to take into account the entry and exit of each slab and the temperature differences which can occur between the head and tail of each slab. This can lead to complicated and expensive measures.
In the casting of slabs with a thickness of approximately 250 mm, the casting machine must be dimensioned to cope with the weight of the large amount of steel present in the machine. However, a casting machine which casts thinner slabs can be constructed to be more than proportionally lighter and therefore also cheaper.
EP-A-0194118 describes a method in which a steel strip with good properties can be produced by rolling it at a temperature of between 300.degree. C. and 800.degree. C. in a conventional 6-stand hot strip finishing train. Because this rolling process takes place in a two-phase region in which austenitic and ferritic material occur alongside each other, it appears that acceptable r-values (see below) are only achievable if the rolling is carried out with a very high speed of deformation. This speed of deformation, expressed as relative elongation per second, must then be at least 300 per second. As a consequence of this it is not practical to couple the rolling and the casting processes to each other.
EP-A-226446 discloses a method of producing thin steel sheets wherein, in one embodiment, after a hot rolling at 1100.degree. to 700.degree. C. of a continuously cast slab 50 mm or less thick, there is performed a lubrication rolling at a temperature between Ar.sub.3 transformation point and 300.degree. C. and at a very high rolling speed of not less than 1500 m/min. Rolling speed as high as 5000 m/min is mentioned. A self-annealing step at 600.degree.-750.degree. C. follows. This lubrication rolling is performed on sheet 2-6 mm thick. It is suggested that this high speed lubrication rolling introduces rolling strain uniformly and effectively to the central portion to the sheet, resulting in improved microstructure. After the high speed rolling, recrystallisation by strain-annealing proceeds at once. Thus reliance is place on a combination of high-speed rolling and self-annealing.
However, such very high rolling speeds create great problems in a process which is truly continuous from continuous casting to coiling. Rolling mills and coilers for such high speeds are expensive, if available, and a continuous casting machine of the capacity required for such a rolling speed is not available.